Magnified telescopic sights, such as rifle scopes and spotting scopes, frequently suffer from a very small exit pupil size. When the exit pupil diameter of the device is approximately the same size or smaller than the diameter of the pupil of the operator's eye, it is challenging to keep the eye within the effective eyebox of the device. Moving the eye out of the eyebox may result in severe vignetting or total loss of the observed scene.
For example, if the diameter of the exit pupil of a telescope is about 2 millimeters (mm) or less, the operator must try to keep his/her head extremely stable, even if the device is equipped with a tripod, in order to keep the eye within the eyebox. Very small, sub-millimeter movements of the operator's head can have significant impact. Therefore, even people who are trained to work with instruments having tight eyeboxes may not be able to keep their heads sufficiently stable, particularly under such circumstances as intensive physical effort (for example, after a fast run), stress, or exhaustion. In addition, it may take several seconds to achieve a proper match between the operator's eye and the eyebox of the device.
Accordingly, various systems have been designed to automatically detect a misalignment between an operator's eye and an eyebox of a telescopic device. These systems typically use imaged eye movements to control other functions of the telescopic device to compensate for the misalignment. For instance, some systems implement feature extraction techniques using image analysis, such as a Hough transform, over the duration of multiple frames of a video output stream.